The present invention relates to heating and cooling apparatus, and more particularly to a defrost subcircuit for use with an air-to-air heat pump.
Air-to-air heat pumps have been in widespread use throughout the United States for many years. These units operate to exchange heat between outdoor air and inside air. For example, a conventional heat pump can operate in either a heating mode during which heat is drawn from the outdoor air and used in heating the inside of the building or in a cooling mode during which heat is drawn from inside the building and released into the outdoor air. Because these systems transfer rather than generate heat, they are generally more efficient than conventional heating and cooling systems.
Air-to-air heat pumps are available in a variety of designs. A typical air-to-air heat pump includes an outdoor air coil unit located outside of the building, an indoor air coil unit located within the building, a plurality of refrigerant lines for interconnecting the indoor and outdoor units, and a control system for controlling operation of the heat pump. In the heating mode, liquid refrigerant enters the outdoor coil unit where it evaporates, thereby drawing heat from the external air into the refrigerant. The gas refrigerant flows from the outdoor coil unit through the refrigerant lines to the indoor coil unit. In the indoor coil unit, the gas refrigerant condenses back into a liquid, thereby releasing heat drawn from the outdoor air into the building. The liquid refrigerant then flows back to the outdoor coil unit to continue the cycle.
In the cooling mode, the process works essentially in reverse. Liquid refrigerant flows into the indoor coil unit where it evaporates to draw heat from the indoor air. The gas refrigerant flows through the refrigerant lines to the outdoor coil unit. In the outdoor coil unit, the refrigerant condenses, thereby releasing heat into the outdoor air. The liquid refrigerant then returns via the refrigerant lines to the indoor coil unit to continue the cycle.
Experience has revealed that when an air-to-air heat pump is operated in the heating mode at close to freezing temperatures, frost can form on the evaporator. This can significantly impair operation of the heat pump. Frost forms on the evaporator when the evaporator draws sufficient heat from the air surrounding the evaporator to freeze the moisture contained in the air. Frosting is typically not a problem at temperatures significantly above or below freezing because at higher temperatures there is enough heat in the air to prevent the moisture from freezing and at lower temperatures the moisture in the air is already frozen so it does not accumulate on the evaporator.
A number of methods have been developed to address the problem of frosting. For example, a number of conventional systems draw heat from inside the building to defrost the evaporator. These systems typically include an indoor coil that draws heat into the refrigerant from inside the building and then pumps the refrigerant through the external evaporator to remove the frost. This approach suffers in that it significantly reduces the efficiency of the heating system because heat is removed from the inside of building to defrost the evaporator. Drawing heat from inside the building can also generate an undesirable cold draft through the duct work. As another example, some systems include an electric heater located next to the evaporator. When the evaporator becomes frosted, the electric heater is turned on to remove the frost. This type of system is also inefficient because it requires operation of a separate electric heater.